1. Field of the Invention
The present invention relates to analysis of the relative power contributions of individual cylinders of an internal combustion engine and, in particular, relates to techniques for determining such power contributions from variations in the engine speed.
2. Description of the Prior Art
The present invention is an improvement of the invention disclosed in copending U.S. application Ser. No. 604,191, filed Oct. 20, 1990, now U.S. Pat. No. 5,182,512. That application discloses a technique for determining cylinder power balance in an internal combustion engine. That technique involves utilizing a contact tachometer to detect the speed variations of an accessible external moving part of an engine, such as a fan belt or the like, the tachometer outputting a signal to an associated engine analyzer, which may be of the type sold by Snap-on Tools Corporation under the designation MT3000 and disclosed in copending U.S. application Ser. No. 587,357, filed Sep. 24, 1990, now U.S. Pat. No. 5,250,935. Such an engine analyzer includes a number of probes, adapted for connection to an associated engine, which can detect, among other things, the ignition events (spark plug firings in the case of gasoline engines or combustion signals/fuel pulses in the case of diesel engines) of each cylinder. The microprocessor of the engine analyzer utilizes the output of the contact tachometer to generate a waveform signal indicative of engine speed variations and displays a single engine cycle of that waveform signal, so that speed variations from one cylinder cycle to the next can be analyzed. In this regard, an engine cycle is the time period between consecutive ignition events of the same cylinder and a cylinder cycle is the time period from the ignition event of one cylinder to the ignition event of the next cylinder in the ignition firing order.
However, that prior system operates to display the waveform signal on a "Vacuum Waveform" display screen of the engine analyzer, which screen, in addition to the waveform, displays at the top of the screen indications of the cylinder ignition events in the ignition order and, at the bottom of the screen, indicates cylinder vacuum events in the vacuum order. Such a screen, designated by the numeral 10, is illustrated, for example, in FIG. 3, wherein the waveform signal is indicated at 11, the ignition event markers are indicated at 12 and the vacuum event markers are indicated at 13. The "Vacuum Waveform" screen is utilized for the waveform display since it is the only suitable display screen available with that engine analyzer, but it is not designed for the purpose of displaying instantaneous engine speed information. Accordingly, that screen displays a number of features and functions relating to vacuum waveforms which the operator has to disregard in viewing a power balance waveform. In particular, the vacuum event markers 13 introduce considerable confusion, since they are out of phase with the ignition event markers 12 by one-half an engine cycle. Accordingly, it is very difficult for an operator to determine the portion of the power balance waveform 11 which corresponded to the response of the engine to any particular cylinder ignition event and careful attention to the user's manual for the contact tachometer is absolutely required in order to understand its use in connection with the "Vacuum Waveform" display screen.